FreeLists / si-list / [SI-LIST] Re: PCB Power Supply Impedance

["Bart Bouma" <bart.bouma@xxxxxxxxx>]

Hi Chris,
The calculation indeed is not difficult. 
In fact it is a (complex-) voltage divider: consisting of the DUT and the 
system impedance.
I assume you measured in a 50 Ohm system.
Since source and load are connected in parallel as seen by the DUT, the 
system impedance seen by the DUT = 25 Ohm.
This 25 Ohm is now our reference, i.e. insertion loss is the ratio in dB 
of the DUT's impedance to this 25 Ohm system-impedance.

The voltage divider:  Vout/Vin = Zdut / (Zdut + 25) .
Ins Loss in dB : 20Log ( Zdut / (Zdut + 25) ). 
(this is the readout of the VNA: S21 in dB).

Following simplified equation is valid as long as the measured impedance 
is much lower than 25 Ohms

Ins Loss in dB : 20Log ( Zdut / 25 ). 

Now the other way around:

Zdut much lower than 25 Ohms:
------------------------------------------------------------
Zdut [Ohm] = 25 * 10^(ins.loss/20)
__________________________________

To be complete: 
the exact equation (to be used when Zdut is not much lower than 25 Ohms) 
is:
--------------------------------------------------------------------------------------------
Zdut  = 25 * [ 10^(ins.loss/20) / (1 - 10^(ins.loss/20)) ]
_____________________________________________________


best regards, Bart Bouma
www.yageo.com





Chris Landrum x311 <clandrum@xxxxxxxxx>

06-01-04 23:12
Sent by: si-list-bounce@xxxxxxxxxxxxx

Please respond to clandrum

 
        To:     "'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
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        Subject:        [SI-LIST] PCB Power Supply Impedance
    Category: 




Hello all,

I am attempting to measure the impedance curve of the power planes of a 
test
board I have built.  I am using a spectrum analyzer to accomplish this and
have obtained a nice capture of dB vs. Freq.  I am having trouble relating
the dB axis to impedance (Ohms).  Can anyone shed some light as to how I
would go about converting the dB to mOhms?

I'm sure this is an easy one, but it is escaping me.

Thanks.
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